This invention relates to a method for the production of a positive electrode for use in the electrolysis of water, and more particularly to a method for the production of a positive electrode to be used in a system for generating hydrogen and oxygen by the electrolysis of an aqueous alkali solution.
The method of electrolyzing an aqueous alkali solution such as, for example, an aqueous solution of potassium hydroxide or sodium hydroxide thereby producing hydrogen at a negative electrode and oxygen at a positive electrode has long been known as a technique suitable for commercial production of hydrogen. In recent years, active studies are being carried out in search for ways of enhancing the efficiency of this method of electrolysis of water. Generally as means for enhancing the energy efficiency in the electrolysis, there are adopted means for improving diaphragms and electrode structures with a view to lowering the electrolytic resistance offered by electrolytes or gas bubbles and means for lowering over-voltages at the electrodes. In the case of the electrolysis of aqueous alkali solutions, the means for lowering overvoltages at the electrodes has attracted special attention.
As measures useful for this purpose, there have been proposed oxide electrodes and nickel electrodes of increased surface area. The former electrodes have their surfaces coated with oxides possessed of high catalytic activity in the reactions causing production of oxygen at the positive electrodes. The electrodes obtained by causing spinel type oxides such as of nickel, cobalt, and iron or perovskite type oxides containing rare earth elements to be compression molded in conjunction with a binder on substrate electrodes can be cited as examples. As examples of the latter electrodes, the so-called surface-enlarged nickel electrodes having their operating surfaces enlarged by means of sintering, flame coating, eluting, etc. can be cited. Concerning the electrolysis of aqueous alkali solutions, however, there is a constant demand for increased efficiency. For example, a strong need is felt for carrying out high-temperature electrolysis at a high current density. When the afore-mentioned oxide type electrodes and surface-enlarged electrodes are used under such harsh conditions, however, they undergo layer separation, structural disintegration, and electrolytic corrosion and cannot be expected to provide the required long periods of service. To be specific, oxide-coated electrodes are deficient in physical strength and surface-enlarged electrodes, though possessed of higher physical strength than the oxide-coated electrodes, tend to entail gradual increase of anodic potential during their operation. These degradations by aging are aggravated in proportion as the temperature is raised and the current density is increased. In the case of an electrolysis performed under the conditions of 50 A/dm.sup.2 and 100.degree. C., for example, an increase of the order of 0.05 to 0.1 V is observed in the anodic potential after 500 hours of operation. This phenomenon is ascribable to the increase in the electric resistance offered by NiO.sub.2, an oxide of higher order to which an oxide of lower order, NiOOH, initially formed on the surface of the nickel electrode is gradually converted. The surface-enlarged nickel electrodes enjoy notable improvement in anodic potential as compared with smooth-faced or reticular nickel electrodes because of their substantially increased surface area. They nevertheless have a disadvantage that so far as their operating surfaces are those of nickel, they inevitably suffer elevation of bath voltage which occurs in consequence of the stabilization of oxides.
The inventors carried out a long-term study in search for a method capable of preventing the aforementioned surface-enlarged nickel electrodes from loss of activity. They have found that these electrodes are most active and suffer least from degradation by aging when they are coated with rhodium, and have completed a method for effecting the coating of the electrodes with rhodium by the electroplating technique.
The electroplating technique, however, entails a variety of difficulties. For example, surface-enlarged nickel electrodes are readily corroded by a sulfuric acid plating bath. Besides, it is extremely difficult to effect uniform plating to the recesses of complicatedly shaped pores formed into the electrodes. To make the matter worse, the increase in the thickness of the coat formed by the plating which is necessary for the purpose of enhancing the fastness of the adhesion of the coat adds to the cost of plating.
The inventors made a study to seek a method for coating nickel electrodes with rhodium by a chemical plating technique instead of the electroplating technique. It has long been held that chemical deposition of rhodium, by nature, is extremely difficult to accomplish. Very few formulas for this chemical deposition have been reported to the art. In fact, an attempt to deposit rhodium on the substrate of a nickel electrode by following the example of the method generally applied to chemical deposition of nickel, cobalt, palladium, or silver, i.e. by formulating a plating bath incorporating a reducing agent such as a hydrogenated boron compound or amine borane in an aqueous solution of antimony or ethylene diamine and treating the nickel electrode in the bath while using varying combinations of pH and temperature conditions fails to provide selective growth of rhodium on the surface of the nickel electrode. For the chemical plating of the nickel electrode with rhodium, therefore, this method is impracticable.
An object of this invention is to provide a method for the production of a positive electrode for use in the electrolysis of water, which method comprises coating a nickel electrode with rhodium by a chemical plating technique.